1. Field of the Invention
The present invention relates generally to medical devices and methods for their fabrication. In particular, the present invention relates to the fabrication of biodegradable endoprostheses, such as stents, having enhanced strength and controlled persistence after implantation.
Stents are generally tubular-shaped devices which function to hold open or reinforce a segment of a blood vessel or other body lumen, such as a coronary artery, carotid artery, saphenous vein graft, or femoral artery. They also are suitable to support and hold back a dissected arterial lining that could occlude the body lumen, to stabilize plaque, or to support bioprosthetic valves. Stents can be formed from various materials, particularly polymeric and/or metallic materials, and may be non-degradable, biodegradable, or be formed from both degradable and non-degradable components. Stents are typically delivered to the target area within the body lumen using a catheter. With balloon-expandable stents, the stent is mounted to a balloon catheter, navigated to the appropriate area, and the stent is expanded by inflating the balloon. A self-expanding stent is delivered to the target area and released, expanding to the required diameter to treat the disease. Stents may also elute various drugs and pharmacological agents.
Of particular interest to the present invention, biodegradable stents and other endoprostheses are usually formed from polymers which degrade by hydrolysis and other reaction mechanisms in the vascular or other luminal environment over time. Usually, it will be desirable to have the endoprosthesis completely degrade after it has served its needed supporting function in the body lumen. Typically, complete degradation will be desired in less than two years, often less than one year, and frequently in a matter of months after implantation. Many biodegradable endoprostheses, however, are persistent for longer than needed, often remaining in place long after the supporting or drug delivery function has ended. The extended persistence of many biodegradable endoprostheses often results from a desire to enhance their strength. The polymer construction materials are often strengthened, such as by incorporating materials having a higher crystallinity, so that they provide desired support but take longer to degrade than would otherwise be desirable.
For these reasons, it would be desirable to provide improved endoprostheses and methods for their fabrication, where the endoprostheses have a controlled strength and persistence. In particular, it would be desirable to be able to enhance the strength of certain biodegradable materials so that they have an improved strength when incorporated into stents and other endoprostheses without substantially lengthening their degradation periods. Moreover, it would be desirable to allow for control of the degradation period in the fabrication process so that an endoprosthesis can be made with different degradation periods while retaining an enhanced strength. At least some of these objectives will be met by the inventions described below.
2. Description of the Background Art
Heat annealing and other treatments of filaments and other components used in stents are described in U.S. Pat. No. 5,980,564, U.S. Pat. No. 6,245,103, and U.S. Pat. No. 6,626,939. Heat treatment of polymeric stent coatings is described in commonly owned, copending application no. PCT/US07/81996, which designates the United States.